1. Field of the Invention
The present invention relates to an improvement of a color separating and composing optical system suitable for color separation and composition in such as recording and reproducing devices of picture images.
2. Description of the Related Art
A color separation prism is well-known as a color separating/composing optical system. Although one advantage of this kind of prism is that the back-focus is short, it has also many disadvantages of spherical aberration, astigmatism, coma, curvature of the field, image distortion, chromatic aberration, etc. It is possible to correct these aberrations with an imaging lens used together with the color separation prism, but its correction is limited.
In addition, as such a lens must be specially manufactured so as to match with the length of the prism, it is unsatisfactory in both performance and cost. Further, it has disadvantages of such as glass distortion and large half-widths of P- and S-polarized lights in dichroic properties. Accordingly, it is especially undesirable when birefringent optical elements are involved.
On the other hand, it is conceivable to devise a color separating/composing optical system having a long backfocus, which employs multiple dichroic mirrors in combination (as shown in FIG. 1). FIG. 1(A) is a view from the sagittal plane and FIG. 1(B) is that from the meridional plane for "G" (green) (the same applies to FIGS. 2-8 and 15).
In FIG. 1, the light having entered from the imaging lens 10 impinges first on a platelike "B" (blue) reflecting dichroic mirror 12 to reflect the "B" light, which forms an image on the image plane 14. On the other hand, the light having passed the mirror 12 impinges on a platelike "R" (red) reflecting dichroic mirror 16 to reflect "R" light, which forms an image on the image plane 18. The "G" light having passed the mirror 16 forms an image on the image plane 20.